A variety of spectroscopic physical and kinetic methods are used to characterize the mechanism(s) of electron transfer in important biological systems. Emphasis is placed on the enzyme complexes of the mitochondrial electron transfer chain and on the metalloflavoprotein, xanthine oxidase. Principal long-term goals are to define the structural and dynamic attributes of the mitochondrial complexes in sufficient detail that hypotheses for the mechanism(s) of energy conservation can be rigorously evaluated. The principal techniques, optical, natural and magnetically-induced circular dichroism and electron paramagnetic resonance spectroscopy will be employed in both equilibrium and kinetic experiments; nuclear magnetic resonance and resonance Raman spectroscopies together with magnetic susceptibility will also be utilized. Major issues to be addressed include: a) The structural and kinetic properties of e components of cytochrome oxidase, the nature of the oxygen bridge and the structure and reactivity of the adduct with hydrogen peroxide. b) The structural basis for the fast to slow transition in purified cytochrome oxidase. c) The structure and reactivity of the iron-sulfur cluster in Complex III. d) The identity and properties of the ligands to the heme iron i the cytochromes of Complex III and the nature of the spectroscopic interaction between the two b cytochromes. e) The kinetic reactivity of Q-depleted Complex III and the relationship between proton translocation and electron transfer in vesicular enzyme. f) Quantitative analysis of the mechanism of xanthine oxidase.